How to convert 15 to 18 digit Id using apex in Salesforce?

Sample Code:

        String fifteenDigit = '0SO90000000PBDu';
Id eighteenDigit = fifteenDigit;        
        system.debug('15 Digit Id is ' + fifteenDigit);
system.debug('18 Digit Id is ' + eighteenDigit);

Output:

Cheers!!!